1. Field of the Invention
This invention relates to optical devices functioning as all-optical logic gates. More specifically, digital optical signals are combined and provided to nonlinear elements such as optical resonators or cavity switches whose resonance frequencies are tuned to produce desired logic output signals.
2. Description of the Related Art
In electronic devices, logic gates composed of transistors comprise the basic elements of digital circuits. Voltage-based inputs are received by the gates, resulting in voltage-based output signals corresponding to the desired logical function.
Interest has begun to emerge in recent years toward development of an optical device that behaves analogously to electronic logic gates. The reason for this interest is that optical signals can potentially travel faster in integrated circuits than electrical signals because they are not subject to capacitance which slows switching speed between logic states. Given the ever increasing demand for faster switching, it is expected that in the future, absent a major technological advance in electronics, use of digital optical devices will become increasingly desirable if not essential.
However, use of optical devices to form integrated logic circuits presents unique challenges. By its nature light propagates and cannot be stored. The ability to represent a logic level stably for as long as may be required becomes an issue. It would thus be desirable to provide optical logic gates that can be used to represent logic states stably using optical signals.
Moreover, there is an established industry using optical components which use primarily amplitude-modulated optical signals in which the amplitude or intensity of light pulses represents digital logic states. Any solution able to store and process data optically should also ideally be compatible with existing optical telecommunications infrastructure.
In some optical modulation schemes, data is represented by more than two amplitude levels. The problem with such an approach is that it requires very stringent control on the amplitudes of the optical signals on which logic operations are performed. For example, in an AND gate, if two pulses are both at high or “1” logic levels represented by an amplitude of “1” in this example, then the output will have an amplitude that is the linear sum of these two levels, or “2”. A “2” is then passed on to the logic gate of the next stage, which must be configured to account for a “2” representing a high logic level and a “1” or “0” representing a low logic level. Thus, the problem of two or more high levels adding becomes more complicated and compounds as logic gates are cascaded. It would therefore be desirable to provide an optical circuit that avoids this problem.
As signals propagate through optical devices, propagation losses become a significant problem that usually inhibits the cascading of optics. Moreover, providing gain to optical signals in a densely integrated substrate currently has technological and practical barriers to being achieved. If restoration of digital optical signals could be managed in another fashion, cascading several optical logic gates would be possible.
Nonlinear optical cavities are typically used to perform all-optical switching. The term ‘nonlinear’ specifically refers to a resonator comprised of a material(s) whose index of refraction depends upon the intensity or power inside the resonator. The incident power depends upon the combination of the input signals, which in turn determines the index of refraction inside the resonator. The resonator's resonance frequency depends upon its index of refraction as follows:
      f    =          qc              2        ⁢                                  ⁢        nL              ,in which f is the resonator's resonant frequency, c is the speed of light, L is the resonator's length, q is any positive integer, and n is the index of refraction. The resonator's unloaded index and length can be adjusted to a slightly different resonant frequency than the input carrier frequency so that only light of sufficient power can increase or decrease the resonator's index of refraction enough to shift the resonator's resonant frequency to equal the incoming carrier frequency. Once the input light resonates within the resonator, the photons have much higher resonator lifetimes and a larger percentage of the input is transmitted through the resonator as an output. The ability of the resonator to readily switch from an opaque state to a transparent state based on a designed amount of input power is why nonlinear cavities are the most common form of all-optical switches.
Although sufficient power can switch a nonlinear resonator to transmit, even greater amounts of input power will further shift the resonator's resonant frequency until it no longer matches the carrier frequency, switching the output off. This behavior has always been considered undesirable, for conventional digital design requires a constant output level regardless of the input level once a threshold is reached. The current thinking and state of the art in research and industry fails to recognize that this behavior could instead be used to a designer's advantage in a way that implementing all-optical logic would be considered much more favorably than it is today.
A nonlinear resonator can also function as the inverse of the detuned resonator described above by having its unloaded resonance frequency equal the input carrier frequency. Inputs with relatively low power will then be transmitted, while inputs of relatively high power will shift the resonator out of resonance and switch the output off. It has not been heretofore recognized that this inverting functionality of a nonlinear resonator is useful if properly utilized in conjunction with other features described above.